The use of various types of air beds for both comfort and therapeutic purposes is known in the art. These have tended to be expensive and somewhat inflexible in their ability to adjust to patient demands. That is, most of the prior art beds are designed to perform a particular purpose and their feel or firmness to a person lying on them is not adjustable.
Hospital patents, in particular, complain about the uncomfortable nature of standard mattresses. Yet the air beds of the prior art are usually too expensive for general use throughout a hospital and in any case are not usually flexible enough in operation to allow comfort adjustment by the patient. There has been a need for a sleep and support surface whose firmness can be adjusted by the occupant.
The following U.S. patents are cited to show air beds whose firmness may be controlled by the occupant: U.S. Pat. Nos. 4,224,706; 4,394,784; and 4,435,864. U.S. Pat. Nos. 3,822,425 and 4,525,885 show forms of air beds specifically designed for medical purposes. U.S. Pat. Nos. 3,653,083; 3,674,019; 4,193,149; and 4,225,989 all show dual bladder air mattresses.
The present invention provides a patient controllable air bed that has built in control features that prevent "bottoming". In the case of some air beds, the user may deflate it so much, in an attempt to obtain a softer rest surface, that he may eventually overcome the supportive effects of the air bed and hit the hard support surface under the mattress. This is clearly undesirable and the present invention prevents this through a differential pressure control system between upper and lower bladders.